A variety of hydrocarbon applications involve the use of cables disposed within a hydrocarbon well. For example, a hydrocarbon cable may be positioned within a well during hydrocarbon production from the well so as to monitor well conditions during the production. A fiber optic core may be present through the cable in order to obtain well condition information such as temperature. In fact, due to the nature of fiber optics a temperature profile of the well may be acquired, with readings from each point along the fiber optic core. Such information may be employed to extrapolate production data useful in the production application. For example, estimates of flow-rate may be calculated by examination of temperature differentials across a temperature profile acquired by such a hydrocarbon monitoring cable.
When employing a hydrocarbon monitoring cable as noted above, the cable may be left in place along several thousand feet of the well and exposed to the environment of the hydrocarbon well for an extended period of time. For example, hydrocarbon production may take place over the course of about 1½ to 5 years with an attempt to leave the cable in place for the duration. In this manner, real-time monitoring of well conditions may be employed. As such, the production application may be modified in accordance with changing conditions within the well.
Unfortunately, prolonged exposure of a hydrocarbon monitoring cable to downhole conditions within the well may leave the fiber optic core susceptible to the formation of defects therein, thus affecting its performance. This may result from exposure to certain downhole substances which behave as fiber-optic defect causing agents leading to optical imperfections within the fiber optic core. For example, hydrogen, present in abundance within an active hydrocarbon well, is prone to attenuate into the fiber optic core reacting with the glass material thereof to form damaging hydroxyl groups. This in turn will lead to internal cracking and deterioration of the fiber optic core resulting in scattering of signal transmissions therethrough. This process may be accelerated by the high temperature of the downhole environment, eventually rendering the cable useless for fiber optic monitoring and communication. In fact, the fiber optic core is generally of a loop configuration, carrying a light signal downhole and providing a return path uphole for measurement and analysis at the well surface. Thus, the opportunity for the formation of such defects along the path of the fiber optic core is twofold from the perspective of the length of the cable.
In order to avoid attenuation of hydrogen or other fiber optic defect causing agents into the fiber optic core, the core may be surrounded by a host of coatings, gels, and/or metal layers acting as shields. A carbon coating, for example, may be particularly effective at shielding an underlying fiber optic core from hydrogen attack. Unfortunately, effective shielding of the underlying fiber optic core may leave the overall profile of the monitoring cable of an impractical size, perhaps occluding the well itself to a significant degree. Furthermore, such shielding may also leave the fiber optic core susceptible to thermally induced defects therein. That is, even where a reduced profile may be achievable, for example, with employment of a thin carbon coating, the addition of this shielding layer introduces a new material for the cable with its own coefficient of thermal expansion. This coefficient of thermal expansion for the newly introduced shielding material may not match that of other material layers surrounding the shielding material, such as a conductor layer or outer polymer jacket. As a result of the mismatching coefficients of thermal expansion between the shielding layer and other outer layers of the monitoring cable, mechanical stress may be experienced by the underlying fiber optic core. As a result, defects in the core may form as the cable is left for an extended period in the generally high temperature downhole environment of the well.
Regardless the type of shielding, the fiber optic capacity of a conventional hydrocarbon monitoring cable of practical sizing will generally be rendered useless in a matter of months if left in place for downhole monitoring. The high pressure, high temperature, hydrogen environment of a typical hydrocarbon well ultimately renders the cable ineffective for continuous fiber optic monitoring of downhole conditions throughout production.